In operation of a gas turbine engine, air at atmospheric pressure is initially compressed by a compressor and delivered to a combustion stage. In the combustion stage, heat is added to the air leaving the compressor by adding fuel to the air and burning it. The gas flow resulting from combustion of fuel in the combustion stage then expands through a nozzle which directs the hot gas to a turbine blade, delivering up some of its energy to drive the turbine and produce mechanical power.
In order to increase efficiency the nozzle has a preestablished aerodynamic contour. The axial turbine consists of one or more stages, each employing one row of stationary nozzle guide vanes and one row of moving blades mounted on a turbine disc. The aerodynamically designed nozzle guide vanes direct the gas against the turbine blades producing a driving torque and thereby transferring kinetic energy to the blades.
The gas typically entering through the nozzle is directed to the turbine at a rotor entry temperature from 850 degrees to at least 1200 degrees Centigrade. Since the efficiency and work output of the turbine engine are related to the entry temperature of the incoming gases, there is a trend in gas turbine engine technology to increase the gas temperature. A consequence of this is that the materials of which the nozzle vanes and blades are made assume ever-increasing importance of elevated temperature capability.
Historically, nozzle guide vanes and blades have been made of metals such as high temperature steels and, more recently, nickel/cobalt alloys. Furthermore, it has been found necessary to provide internal cooling passages in order to prevent oxidation. It has been found that ceramic coatings can enhance the heat resistance of nozzle guide vanes and blades. In specialized applications, nozzle guide vanes and blades are being made entirely of ceramic, thus, accepting even higher gas entry temperatures.
Ceramic materials are superior to metal in high-temperature capability and have a low linear thermal expansion coefficient. But, on the other hand, ceramic materials have negative drawbacks such as low fracture toughness.
When a ceramic structure is used to replace a metallic part or is combined with a metallic one, it is necessary to avoid excessive thermal stresses generated by an uneven temperature distribution or the difference between their linear thermal expansion coefficients. The ceramic components' different chemical composition, physical property and coefficient of thermal expansion to that of a metallic supporting structure result in undesirable stresses. A major portion of these stresses is thermal stress, which will be set up within the nozzle guide vanes and/or blades and between the nozzle guide vanes and/or blades and their supports when the engine is operating.
Furthermore, conventional nozzle and blade designs which are made from a metallic material are capable of absorbing or resisting these thermal stresses. The chemical composition of ceramic nozzles and blades do not have the desired characteristics to absorb or resist the thermal stresses. If the stress occurs in a tensile stress zone of the nozzle or blade a catastrophic failure may occur.
The present invention is directed to overcome one or more of the problems as set forth above.